Iraq's request to the U.S.-led coalition for air strikes in the campaign to retake Tikrit from Islamic State insurgents is "imminent", a senior diplomat from a Western nation that is part of the coalition told Reuters on Tuesday.

If the coalition accepts the request, it would see by far the biggest collaboration so far against the militants by Iraqi forces, the Iranian-backed paramilitaries and their Iranian advisers on the ground, and the United States and its allies.

There was no immediate confirmation from the Iraq government about the request, which the diplomat said would be positively received. "Once that's happened (that) we have gotten the request, we will do whatever we are asked to do," the diplomat said, speaking on condition of anonymity.

The coalition has been absent so far from the Tikrit campaign launched three weeks ago, the largest to be undertaken by the Iraqi forces and Iranian-backed Shi'ite Muslim militias since Islamic State overran a third of the country last year.

A military official within the coalition told Reuters on Tuesday that the coalition began providing intelligence, surveillance and reconnaissance for the Tikrit operation on March 21 after a request from the Iraq government. "The U.S. is now providing that support," he said.

More than 20,000 troops and allied Shi'ite paramilitary groups are taking part in the offensive which has been on pause for nearly two weeks after they suffered heavy casualties on the edge of the city, 160 kilometers (100 miles) north of Baghdad.

The Iraqi military had lobbied for U.S.-led coalition air strikes while Shi'ite paramilitary forces opposed such a move. One militia leader, Hadi al-Amiri, boasted three weeks ago that his men had been making advances for months without relying on U.S. air power.

Seeking momentum

The mainly Sunni city of Tikrit, Saddam Hussein's home town, was seized by Islamic State in the first days of their lightning strike across northern Iraq last June.

If Iraq's Shi'ite led-government retakes Tikrit it would be the first city wrested from the Sunni insurgents and would give Baghdad momentum for a pivotal stage of the campaign: recapturing Mosul, the largest city in the north.

The diplomat added any strikes in Tikrit would be based on the model in the north where the coalition has carried out air strikes in tandem with the Kurdish pershmerga forces.

"I guess (the strikes) will be targeted against machine gun posts" and strike fighters if they fled to open ground, the diplomat said.

The most prominent Iranian military officer seen on the battlefield during the Tikrit offensive is Major-General Qassem Soleimani, commander of the al-Quds brigade of Iran's elite Islamic Revolutionary Guard Corps (IRGC).

"Iranians fighting alongside the coalition is not a bad thing," the diplomat said, comparing the potential U.S.-Iranian alliance against Islamic State to the Western allies and Soviets battling Nazi Germany in World War Two.

"Strange bedfellows, isn't it?" he said. "What did Churchill say? 'I would sup with the devil himself if it defeated Hitler.'"